Peak-end rule
According to the peak-end rule, we judge our past experiences almost entirely on how they were at their peak (pleasant or unpleasant) and how they ended. Virtually all other information appears to be discarded, including net pleasantness or unpleasantness and how long the experience lasted.In one experiment, people were subjected to loud, painful noises. In a second group subjects were exposed to the same loud, painful noises as the first group, but after which was appended somewhat less painful noises. This second group rated the experience of listening to the noises as much higher than the first group -- despite having been subjected to the same discomfort as the first group but with a little more added on.
This heuristic was first suggested by Daniel Kahneman and others. He argues that because people seem to perceive not the sum of an experience but its average, it may be an instance of the representativeness heuristic.
See also: list of cognitive biases.